1. Field of the Invention
The present invention relates to an on-board apparatus installed in a vehicle.
2. Description of Related Art
There is an apparatus known in the related art capable of calculating the grade of the road ahead of the subject vehicle based upon sets of mesh altitude data, each stored in correspondence to a specific area among areas set by partitioning a map into mesh areas (see Japanese Laid Open Patent Publication No. 2001-50743). This apparatus calculates the grade of the road ahead of the subject vehicle by setting “virtual” points over predetermined distance intervals along the forward direction on the road upon which the subject vehicle is currently traveling, calculating a grade of the road segment between the current subject vehicle position and each virtual point with using the mesh altitude data of the area where the particular virtual point is located as the altitude data for the virtual points, and then averaging the grades of the road segments thus calculated.
The mesh altitude data used in the apparatus disclosed in patent literature 1 are set in units each corresponding to a predetermined area, e.g., a 10 m×10 m square area. This means that the mesh altitude data stored in correspondence to the mesh area where a given virtual point is located do not necessarily match the actual altitude of the virtual point, giving rise to the distinct possibility that an error in the road grade calculation results will occur. In addition, while an average road grade of the road ahead of the subject vehicle can be calculated by averaging the grades of the road segments between the subject vehicle position and the various virtual points through this method, the road grade at a specific geographical point present ahead of the subject vehicle cannot be calculated.
As a means for addressing the issues of the road grade calculation method in the related art described above, a method of road grade calculation is recently proposed whereby the altitude values at road points in various regions are measured in fine increments and the road grade at a specific geographical point present ahead of the subject vehicle is calculated by using altitude data set based upon the measured values. However, this road grade calculation method, achieved in conjunction with altitude data set in fine increments, is bound to be greatly affected by any error in altitude measurement. For instance, a road grade calculated based upon two sets of altitude data set for two points separated from each other by 1 m with either set of the altitude data manifesting a measurement error of 10 cm will contain ±10% error relative to the percentage value representing the actual road grade. Namely, assuming that the actual road grade is, for instance, 5%, the road grade resulting from the calculation will be off by as much as 15% or −5%, which is vastly different from the actual road grade.
An object of the present invention is to enable accurate calculation of the road grade at a specific geographical point present ahead of the subject vehicle by minimizing the adverse effect attributable to an altitude measurement error occurring as described above.